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Electronics Schematics - H

Hartley Oscillators Circuits

CK-3V FM Transmitter

Components of a simple bandpass filter

design and construction of radio frequency oscillators

Electronic Canary:  This circuit is a modified hartley oscillator with a couple of extra components included. The transformer is a small audio transformer, type LT700. The primary is center tapped with an impedance of 1Kohms at 1KHz . The secondary has an impedance of 8 ohms. The inclusion of R1 and C1 give this oscillator its characteristic "chirp". As the 100u capacitor charges via the 4.7K resistor, R1 the bias for the transistor is cut off. This causes the oscillation to stop, the capacitor discharges through the base emitter circuit of the transistor and oscillations start again. Altering these components alters the frequency of the chirp. The chirp is also voltage dependent. When the push button switch is operated the 100u capacitor is charged. When its released, the oscillation decays and the chirp becomes faster.

I.F. Amplifier:  The I.F. transformer primary has 18 turns, the secondary winding has 4 turns. The capacitors across the IFT primaries are 82pF. The input/output transformer has 12 turns, tapped at 3 turns from ground. This transformer is wound on a ferrite core. The mosfets are 3SK45's. The diodes in the product detector are 1N34's. I use a six pole SSB filter from a scrap CB. The centre frequency is 7.8MHz. The -6db bandwidth is about 2.5KHz.

Regarding Tapped Capacitor Impedance Transformation in LC Bandpass Filters:  We often use a pair of capacitors to match impedances at the termination ends of LC bandpass filters.  The circuit consists of a shunt capacitor at the termination followed by a series capacitor connecting to the high Z end of a parallel tuned circuit.

Resonator BFO Circuit:  This circuit was used to stop all the BFO drift. The circuit is extremely stable. Turn the receiver off, and then on at any time and temperature, the BFO frequency is exactly the same.

series regulator with Q900 :  This is a series regulator with Q900 being the control element, Q901 a driver, and Q902 an error amp. ZD900 forms the emitter reference voltage source. Since the generated high voltage and other voltages are linked by means of the magnetic field of T900, any change in H.V. will be reflected back to all of the other voltages.

Sine Wave Oscillators:  This oscillator gives a really beautiful sine wave, and is an excellent choice for a precision audio oscillator. Its characteristic feature is the RC network consisting of R and C in series with a parallel combination of R and C, as shown in the circuit diagram below. The resistors and capacitors can be different in value, but it is much simpler to take them equal, and nothing of value is lost.

VFO/Buffer:  It's basically a standard Hartley oscillator, followed by Roy Lewallen's buffer (page 14.20 of the 2001 Handbook). Output is +7 dBm into 50 Ohms. Don't be tempted to add a gate diode, this circuit doesn't need it, and it will degrade the phase noise performance, according to Ulrich Rohde. It should be suitable for any frequency up to 10 MHz or more (depending on how good you are at making drift-free oscillators) and may be tuned with a suitable capacitor or varicap tuning diode.

Was That Really A Wildlife Tag?:  In wildlife management, very few tracking transmitters send some form of "station identification." Radio tags for birds and small mammals can't. These little devices must put out a signal to a ground range of a mile or so for up to a year or more, yet weight only a few grams, including battery. To do that, most of them consist of simple blocking or relaxation oscillators with only a few discrete components. You may hear the term "squegging oscillator" used to describe them.

What I had to do to stabilize a VFO:


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